1. Field of the Invention
The invention relates to optically transmissive silica-containing compositions which exhibit a small change of refractive index with respect to changes in temperature, dn.sub.g /dT. The gels are derived from silica sols dispersed in liquid organic polymers. The invention also relates to optical systems comprising a plurality of elements that are optically coupled by means of the inventive composition. An especially preferred embodiment of the invention is use of the composition in optical fiber splices and/or connectors to simultaneously align and join optical fibers.
2. Background of the Art
A persistent problem relating to prior art refractive index matching materials (IMM) used to optically connect optical elements such as optical fibers and waveguides is the sizable change in refractive index that results from changes in ambient temperature dn.sub.g /dT--a value that is also referred to as the temperature coefficient of the refractive index. The change in refractive index causes a variety of problems well known to those schooled in the art, which are further aggravated by the accompanying thermal expansion or contraction of the IMM, the connectors, and the splices forming the interconnections. It is usually the case that the coefficient of thermal expansion is larger in the IMM than in the joined optical elements. Index matching compositions (IMC) as described within the context of this invention are mixtures of two or more materials, often gels, that are used primarily in splices for simultaneously aligning and joining optical fibers or waveguides.
Prior art IMCs tend to consist of liquids (often silicone oils) containing a low concentration of hydrophobic fumed silica. The silica acts as a thixotropic agent to prevent material flow. The gel, having a typical viscosity in the range 1.5-2.0 kPascal-second at 25.degree. C., is filtered to remove particles of size greater than 200 nanometers which would scatter light. At the concentrations used (about 10% by weight), the fumed silica provides no significant reduction in dn.sub.g /dT, where n.sub.g is the refractive index of the IMC. For these conventional silicone based IMCs, dn.sub.g /dT is usually in the range of -3.3 to -3.7.times.10.sup.-4 /.degree.C. For optical fibers made of silica glass, dn.sub.f /dT, is typically of the order of -10.sup.-6 /.degree.C., where n.sub.f is the refractive index of the fiber. The state of the art at present requires IMCs with dn.sub.g /dT smaller in magnitude than -3.3.times.10.sup.-4 /.degree.C., as reduction of this value would result in improved signal transmission and reduced reflective losses through the splice over the desired range of operating temperatures. A large number of patents refer to the use of specific index matching materials such as index matching fluids, gels, adhesives, cements, and films in fiber optic connectors. Of interest are U.S. Pat. Nos. 5,013,123; 4,991,929; 4,856,865; 4,784,456; 4,729,619; and 4,448,483.
U.S. Pat. No. 5,013,123 describes stamped precision lightguide interconnect centering elements made from deformable metals (e.g., aluminum) or polymers that can be formed to define three surfaces that align and clamp the ends of two optical fibers, forming a butt splice therebetween. The text at column 6, lines 21 through 25, describes the use of a gel which has index of refraction matching characteristics similar to the fiber core to improve the continuity of the transmission through the splice. The position of the gel is shown in FIG. 13, ref. numeral 100.
The addition of particulate silica as a reinforcing agent to improve the mechanical properties of silicone polymers is well known in the art. For example U.S. Pat. Nos. 3,996,187 and 3,996,189 (both to Travnicek) describe fumed silica reinforced silicones which have utility in making contact lenses when the silica level is kept between 5 and 20% by weight. Such levels of fumed silica are too low to have an appreciable effect on the temperature dependence of the refractive index. Fumed silica (also called pyrogenic silica) is a colloidal silica powder usually made by condensing silica from a vapor phase at elevated temperatures. The colloidal particles in fumed silica are not electrostatically charged and are therefore clustered into large aggregates. See The Chemistry of Silica by R. K. Iler, 1979, p.464, John Wiley and Sons for greater detail on this material.
U.S. Pat. No. 4,008,198 (Krohberger et al.) teaches that sheets of highly transparent reinforced elastomers with improved tear strength can be obtained by mixing (1) a nitrogen compound containing at least one triorganosilyl group of a particular type, (2) a hexaorganodisilazane, (3) silicon dioxide having a surface area of at least 50 m.sup.2 /g and (4) a polydiorganosiloxane of viscosity higher than 500,000 cS, and kneading the mixture at 150.degree. C. under vacuum until there is no evidence that nitrogen compounds are being evolved.
U.S. Pat. No. 4,418,165 (Polmanteer et al.) describes silicone compositions with improved tensile strength comprising a hydrophobized "reinforcing" silica gel obtained by the alkaline hydrolysis in ammonium hydroxide of a mixture of tetraalkyl orthosilicate (tetraalkoxy silanes) and a hydrophobizing agent. The silica gel is laboriously milled or high shear mixed with a silicone gum to breakup large agglomerates into smaller aggregates. We have found that when mixed with fluids, this form of silica hydrogel yields highly rigid, embrittled composites with inferior luminous transmission when the silica level exceeds 20% by weight.
U.S. Pat. Nos. 4,898,755 and 4,851,270 describe optically clear inorganic-organic compositions prepared from porous monoliths of hydrolyzed alkoxy silanes and organic compounds exhibiting nonlinear optical responses.
An ideal index matching composition would possess the following four optical characteristics: 1) It would be 100% transmissive at the wavelengths of light employed, 2) Its refractive index would be the same as (i.e matched to) that of the optical element (e.g., a fiber) at this wavelength, 3) The refractive index of the optical element and the IMC would possess the same temperature dependency (i.e., the same change in indices of refraction for the same temperature changes) and 4) The refractive index of the optical element and the IMC would possess the same wavelength dependency.
In reality such an ideal IMC is presently unattainable. 100% transmissivity is virtually impossible to achieve due to light scattering caused by particle aggregation in the gel and/or the mismatch between the refractive indices of the colloidal particles in the IMC and the dispersing polymer phase. In addition, although the refractive indices of the IMC and the optical element may exactly match at one temperature, they cannot be made identical over a working temperature range because the temperature and wavelength dependence of the refractive index of the optical element has been found to be different from the temperature and wavelength dependence of the refractive index of the IMC.
In addition to its optical characteristics, the viscosity of the IMC is important The index matching gel must possess viscosity characteristics that allow it to flow through small orifices during the application of the IMC to the splice element and to wet the surfaces to be connected. This viscosity requirement therefore virtually precludes the use of fumed silica or silica gels at levels much higher than 10% by weight because both of these forms of colloidal silica are extremely efficient thickening agents that would hinder both the prefiltration of the IMC and its application to the connector device.
Prior art IMMs have not sufficiently satisfied all of the above desirable characteristics simultaneously. In particular, a persistent problem relating to IMMs of the prior art is the sizeable change in the materials properties resulting from changes in ambient temperature, i.e. the refractive index, the viscosity, and dimensional changes.
No prior art is known which discloses the beneficial effect of high concentrations of finely dispersed small particle silicas for reducing dn.sub.g /dT in refractive index matching compositions while maintaining the required low viscosity and high level of light transmissivity.